Differing interactions between hexamethonium and tubocurarine, pancuronium or alcuronium at the neuromuscular junction.

The action of hexamethonium has been investigated in the rat phrenic nerve-hemidiaphragm preparation, alone and in combination with the neuromuscular blocking agents tubocurarine, pancuronium and alcuronium. Hexamethonium alone in concentrations between 3.55 x 10(-3) and 7.1 x 10(-3) mol litre-1 produced neuromuscular blockade in a dose-dependent manner. Low concentrations of hexamethonium antagonized the neuromuscular blocking effect of all three neuromuscular blocking drugs, less with tubocurarine than with the other two. Increasing the concentration of hexamethonium produced potentiation of the neuromuscular blocking effect, this being greater with tubocurarine than with either pancuronium or alcuronium. The cholinesterase activity in rat diaphragm homogenates was inhibited by hexamethonium. This inhibition was only significant at concentrations greater than those which resulted in antagonism and cannot, therefore, explain the observed antagonism. The mechanism of these observations is discussed with respect to the known behaviour of a combination of antagonists acting within a receptor system.